1. Field of the Invention
The present invention relates to a battery system with a plurality of battery cells, which are rectangular batteries, held in a stack by fastening components.
2. Description of the Related Art
As shown in FIG. 1, a battery system with a plurality of stacked battery cells, which are rectangular batteries, has a pair of endplates 905 connected by bolts 906 to dispose batteries 901 in a stack between the endplates 905. (Refer to Japanese Laid-Open Patent Publication No. H03-32364A (1991).) In the battery system cited in JP H03-32364A as shown in FIG. 1, the endplates 905 are provided with lateral protrusions 908 on both sides for insertion and connection of the bolts 906. The bolts 906 are inserted through the lateral protrusions 908 and nuts 907 are threaded onto the ends to hold the endplates 905 in place. A battery system with this structure has the drawback that depending on operating conditions, battery performance can deteriorate. In particular, this type of battery system has the drawback that since lithium ion batteries are used as the rectangular batteries, depending on operating conditions, battery internal resistance can increase and reduce output. This is because the lateral protrusions can easily deform and expansion of battery external cases cannot be reliably prevented. In a lithium ion battery, internal battery electrode elements can expand during over-charging or during normal charging and discharging resulting in wider separation between positive and negative electrodes. If this condition occurs, the distance ions, which move between positive and negative electrodes, must traverse increases and internal resistance increases. In particular, compared to other batteries such as nickel cadmium batteries, lithium ion batteries have large internal resistance per area of the opposing electrodes. To reduce internal resistance, lithium ion battery positive and negative electrode plates are made thin, the gap between the electrodes is reduced, and opposing electrode area is increased. Since the gap between positive and negative electrodes is narrow to reduce internal resistance, the internal resistance can become large even if electrode expansion results in only a small increase in the gap between electrodes. If the internal battery resistance becomes large, the amount of current that can be extracted is lowered and the power output is also reduced. Since a battery system with a plurality of stacked rectangular batteries is used in applications demanding high output such as in an automobile, reduction in output is a serious drawback.
Further, the battery system of FIG. 1 has the drawback that if the rectangular batteries 901 expand, the bolts 906 can contact the sides of the rectangular batteries 901. This is because if the rectangular batteries 901 expand and the lateral protrusions 908 bend inwards, bolts 906 inserted through those lateral protrusions 908 will curve inwards. In the stack of rectangular batteries, there is a potential difference between the external cases of adjacent batteries. Consequently, if a bolt contacts a plurality of rectangular battery external cases, short circuit current flow will detrimentally affect the batteries and compromise safety.
A battery system with stacked batteries held by metal bands instead of bolts has been developed. (Refer to Japanese Laid-Open Patent Publication No. H05-343105A (1993) and 2001-507856A.)
In JP H05-343105A, metal bars are provided at both ends of a stack of rectangular batteries, and metal bands are fastened at both ends to those metal bars via set screws. In a battery system of this structure, the metal bars can easily deform with rectangular battery expansion, and it is difficult to reliably prevent rectangular battery expansion and battery performance degradation. JP 2001-507856A cites a battery system provided with a pair of end-panels disposed at the ends of a stack of rectangular batteries, and metal bars are welded to those end-panels to hold the stack. This battery system has the drawback that since the metal bars are attached by welding, it is difficult to hold the rectangular batteries with a properly adjusted amount of compression.
To correct the drawbacks described above, the present applicant developed the battery system shown in FIG. 2. (Refer to Japanese Laid-Open Patent Publication No. 2008-282582A.) This battery system is provided with a battery block 92 having a plurality of stacked battery cells 91 that are rectangular batteries, and fastening components 93 that hold the battery cells 91 of the battery block 92. The fastening components 93 are in turn provided with a pair of endplates 94 disposed at the ends of the battery cell 91 stack, and metal bands 95 that connect the ends of the battery cell 91 stack and retain it in a compressed state.
The battery system described above holds a stack of rectangular batteries in an ideal state with the pair of endplates and metal bands, and can prevent battery performance degradation during use. In particular, by disposing the metal bands close to battery block surfaces, namely by disposing the metal bands in close proximity to battery cell outer side-walls, endplate size can be essentially equal to the outline of a battery cell and the endplates can be solidly fastened with the metal bands. In addition, by minimizing metal band and endplate protrusion from the battery block, battery system outline can be made compact.
However, this battery system has the drawback that the metal bands can easily contact and short circuit battery cell outer side-walls. This is because the metal bands are put in close proximity to battery cell outer side-walls to achieve the excellent characteristics described above. A battery system with stacked battery cells is not used with all the battery cells connected in parallel, but rather has series-connected battery cells to increase output voltage. A voltage difference arises between the external cases of adjacent battery cells that are connected in series. The external case of a battery cell is connected to the positive or negative electrode, or it is not connected directly to an electrode and attains a potential between that of the positive and negative electrodes. For example, for a lithium ion battery used as a battery cell, the external case assumes a potential that is intermediate to that of the positive and negative output terminals. This is because the external case connects to the positive and negative electrodes through the electrolyte solution. Since there is a voltage difference between adjacent battery cells, if a metal band in close proximity to the outer side-walls contacts those external cases, a large short circuit current will flow. Further, if an external case contacts a metal band, it can cause leakage current.
The present invention was developed with the object of further correcting the drawbacks described above. Thus, it is an important object of the present invention to provide a battery system that can reliably prevent short circuits between the outer side-walls of battery cells and the metal bands while putting the metal bands in close proximity to battery cell outer side-walls. It is another important object of the present invention to form insulating walls, which insulate the metal bands from battery cell outer side-walls, in single-piece construction with the insulating separators, which are made of plastic and insulate adjacent battery cells; and thereby provide a battery system that can reliably insulate the metal bands from the battery cells by disposing the insulating walls in proper positions, without shifting position, via the insulating separators.